Using the Safety Interlock

You can integrate the PXIe-4136 into a safety interlock system.

What the Safety Interlock Does

When integrated into an appropriate system, the safety interlock protects users from hazardous voltages. Correct use of the safety interlock system is required to output up to the maximum voltage of the instrument; you can still operate the instrument at lower voltages without using the safety interlock.

The PXIe-4136 PXI source measure units include this safety interlock functionality.

A properly integrated safety interlock system applies limits to both of the following:

  • The voltage setpoint: the combination of voltage levels and limits you can set
  • The output terminal voltage: the voltage measured between the Output HI and Output LO terminals on the instrument

The safety interlock circuit on NI SMUs has two possible states: closed or open. You should design the safety system to take advantage of these states in order to protect operators.

  • Closed—The safety interlock circuit is complete; the safety interlock is not restricting the voltage levels and limits of the channels on the instrument.

    Hazardous voltages up to the maximum voltage of the instrument are allowed on the output terminals.

    In a physical sense, the interlock is defined as closed when the input terminal of the safety interlock is shorted to the ground terminal of the safety interlock.

  • Open—The safety interlock circuit is open; the safety interlock is restricting the setpoint voltage and output terminal voltage of each channel on the instrument to specific maximums.

    Programming a channel to exceed one of these maximum thresholds while the interlock circuit is physically open results in an interlock error. Hazardous voltage levels that appear on the output terminals of a channel while the interlock is open cause all output channels of the instrument to shut down, which stops voltage generation.

    You should design your safety system so that the safety interlock terminal is open when the output connections are accessible.

    • Note Be aware of any other voltage sources in your test system that exceed the safety interlock thresholds in your test system and of capacitance that could remain charged at hazardous voltages in the system, and take the appropriate safety precautions. The safety interlock of NI-DCPower instruments protects operators only from voltages generated by the instrument, not from voltages external to the instrument that may be present in systems.

    Once integrated into the overall safety interlock system, you can use the NI-DCPower API, which controls NI SMUs, to use the detected state of the safety interlock as part of your program.

    Protection Thresholds when the SMU Safety Interlock is Open

    The safety interlock system of supported NI SMUs is designed to protect operators from high voltages when the interlock circuit is open. To provide this protection, the NI-DCPower SMU driver generates errors that interrupt generation of voltages above specific thresholds.

    The NI-DCPower instrument driver generates safety interlock errors with specific behavior when either the maximum voltage setpoint or the maximum output terminal voltage exceeds the following limits.

    Table 15. NI SMU Safety Interlock Voltage Thresholds and Behavior
    Threshold Definition Interlock Error Occurs At Behavior
    Maximum voltage setpoint The highest voltage level or voltage limit you can set when the safety interlock is open. Voltage > ±40 V DC
  • Single Point source mode—NI-DCPower generates an error, and channels of the instrument shut down.
  • Sequence source mode—NI-DCPower continues to generate the prior valid sequence point on the output and does not generate at the excessive level or limit.
    • Maximum output terminal voltage The highest voltage output that can be present between the Output HI and Output LO terminals when the safety interlock is open. Voltage > ±(42 V pk ±0.4 V) NI-DCPower generates an error, and channels of the instrument shut down.

    Integrating the NI-DCPower Safety Interlock System

    To take advantage of the protections that the safety interlock of an NI-DCPower instrument affords, you have to integrate the instrument into a larger safety system that can make use of the interlock.

    Consult the following to set up a safety interlock system for your NI-DCPower instruments.
    1. Follow Safety Guidelines for Safety Interlock System Implementation to develop a correctly functioning safety interlock system for your NI-DCPower instruments.
    2. Complete Installing Safety Interlock Cabling to correctly connect your instruments to the safety interlock system.
    3. Complete Testing the Safety Interlock to verify the functionality of the system, and continue to follow the recommended test interval.

    Safety Guidelines for Safety Interlock System Implementation

    The safety interlock circuit limits the outputs of the PXIe-4136 to a safe state, regardless of the programmed state of the instrument. To make use of this functionality, you must incorporate the instruments into an appropriate enclosure system that follows all provided guidelines.

    Caution Hazardous voltages of up to the maximum voltage of the PXIe-4136 may appear at the output terminals if the safety interlock terminal is closed. Open the safety interlock terminal when the output connections are accessible. With the safety interlock terminal open, the output voltage level/limit is limited to ±40 V DC, and protection will be triggered if the voltage measured between the device HI and LO terminals exceeds ±(42 V peak ±0.4 V).
    Notice Do not apply voltage to the safety interlock connector inputs. The safety interlock connector is designed to accept passive, normally open, contact closure connections only.

    Guidelines for System Design

    • Do not defeat the safety interlock by shorting the safety interlock pins directly at the connector under any circumstances.
    • Confirm on a regular basis that the safety interlock is functioning by performing a safety interlock test.
    • Connect the safety interlock terminal to a limit switch of a test fixture or shielding box.
    • Install mechanical detection switches that open the safety interlock circuit when the operator attempts to access the test fixture, disabling the hazardous voltage ranges of the instrument.
    • Ensure the mechanical detection switches close the safety interlock circuit only when the operator has properly closed all entry points to the test fixture enclosure, enabling hazardous voltage ranges on the instrument.

    The PXIe-4136 is capable of generating hazardous voltages and working within hazardous voltage systems. It is the responsibility of the system designer, integrator, installer, maintenance personnel, and service personnel to ensure the system is safe during use.

    • Ensure operators cannot access the PXIe-4136, cables, the device under test (DUT), or any other instruments in the system while hazardous voltages are present. Operator access points can include, but are not limited to, guards, gates, sliding doors, hinge doors, lids, covers, and light curtains.
    • If using a test fixture enclosure, ensure that it is properly connected to safety ground.
    • Ensure that the PXIe-4136 is properly secured to the chassis using the two front panel mounting screws.
    • Double insulate all electrical connections that are accessible by an operator. Double insulation ensures protection if one layer of insulation fails. Refer to IEC 61010-1 for specific insulation requirements.

    When the NI SMU is operating at Voltage > ±40 V DC, your system should comply with these guidelines:

    • Ensure the test system provides a contact closure between the safety interlock terminal and the ground terminal of the safety interlock connector.
    • Ensure the contact closure is open when your test fixture or shielding box allows touch access to any conductors that carry hazardous voltage.

    Mechanical Detection Switch Recommendations

    • Use high-reliability, fail-safe, normally open mechanical detection switches on all access points to the test fixture enclosure.
    • Use two normally open switches wired in series so that a single switch failure does not compromise safety protections.
    • Isolate switches so the operator cannot trigger or bypass the switches without the use of a tool.
    • Ensure the switches' certifications meet your test application requirements. NI recommends UL-certified safety switches to ensure reliability.
    • Install the switches in accordance with the switch manufacturer specifications.
    • Test the switches periodically to ensure proper implementation and reliability.

    Guidelines for System Operation

    To ensure a system containing the PXIe-4136 is safe for operators, components, or conductors, take the following safety precautions:

    • Ensure proper warnings and signage exist for workers in the area of operation.
    • Provide training to all system operators so that they understand the potential hazards and how to protect themselves.
    • Inspect connectors, cables, switches, and any test probes for any wear or cracking before each use.
    • Before touching any of the connections to the high terminal or high sense on the PXIe-4136, discharge all components connected to the measurement path. Verify with a DMM before interaction with connections.

    Archetypal Safety Interlock System

    For example, the following figure illustrates a typical safety interlock circuit system connection.

    Figure 21. Archetypal Safety Interlock System Design

    1378

    1. Safety interlock input
    2. Safety interlock ground
    3. Safety interlock pass-thru: input
    4. Safety interlock pass-thru: ground
    5. Safety interlock input connector
    6. Test fixture enclosure
    7. Operator access door
    8. Mechanical detection switches, actuated by operator access points

    In this archetypal system, the voltage output of the instrument is limited when the operator access door is open. The mechanical detection switches are open when the door is open, which opens the safety interlock circuit in turn and limits the output voltage. When the door is shut, the safety interlock circuit is completed (closed), and the instrument(s) connected to the circuit can output up to their maximum voltage.

    Installing Safety Interlock Cabling

    Safety interlock cabling connects the safety features of your systems to the interlock circuit of supported NI-DCPower instruments.

    Before installing safety interlock cabling, install the instrument(s) in your chassis, as described in the getting started process for your instrument.

    This procedure assumes you have already designed a test system to make use of the safety interlock.

    Correct usage of the safety interlock is required to output voltage above a defined threshold from your instrument.

    You can use either NI cabling or generic cabling to make safety interlock connections.

    Based on your choice of cabling, follow the appropriate steps to wire the safety interlock subsystem of your test system.
    • Using the Safety Interlock
    • Installing Generic Safety Interlock Cabling

    Installing the Safety Interlock Cable

    This procedure applies to installing the 8 in. and 48 in. Safety Interlock Cable for PXIe-4136.

    These cables include an integrated safety interlock input connector for connecting to the instrument front panel, and an unterminated end to wire the cable to the test system enclosure at the user access points. The cables are available in the following lengths.

    Table 16. Safety Interlock Cable for PXIe-4136
    Length NI Part Number Connection Distance Unterminated End Characteristic
    8 in. 142998-08 1 to 4 chassis slots Pre-stripped
    48 in. 142998-48 >4 chassis slots, or connecting to another chassis Unstripped

    Complete the following steps to wire the safety interlock subsystem using the Safety Interlock Cables for PXIe-4136.

    1. Ensure the AC power source is connected to the chassis before installing the connector.
      The AC power cord grounds the chassis and protects it from electrical damage.
    2. Power off the chassis.
    3. Touch any metal part of the chassis to discharge static electricity.
    4. Prepare and connect a 48 in. cable to the system safety relay you have designed for your interlock implementation.
      1. Measure and mark your strip length on the cable.
      2. Use an insulation strip tool to strip back the insulation to the appropriate length.
      3. Wire the cable to the test system enclosure at the user access points as instructed in any documentation for the enclosure.
    5. To extend your safety interlock circuit to additional instruments, connect cabling of sufficient length to the interlock connector in the previous step.
      NI recommends the 8 in. cable for a distance of one to four chassis slots, and the 48 in. cable for four or greater chassis slots or connecting to another chassis.
      Note If you use a 48 in. cable, measure and strip the cable as listed in the previous step. You can cut and strip 48 in. cables to shorter lengths as appropriate for your system.
      1. Connect the red safety interlock signal wire from a second interlock cable into pin 3 on the interlock connector of the first cable.
      2. Connect the black safety interlock ground wire from a second interlock cable into pin 4 on the first interlock connector.
      3. Repeat with additional cables until you have connected enough safety interlock connectors for as many instruments as you need.
      Figure 22. Safety Interlock Pass-Thru Connection

      1378

    6. After inserting all of the cables, inspect for loose strands.
    7. Tighten any retention screws on the safety interlock input connectors to hold the cabling in place.
    8. Attach the safety interlock input connectors to the instruments.
    9. Power on the chassis.
    Once you have properly connected safety interlock cabling within your system, continue with Testing the Safety Interlock System.

    Installing Generic Safety Interlock Cabling

    This procedure applies to installing generic safety interlock cabling with NI SMUs.

    You can make safety interlock connections with any unshielded twisted-pair cabling that meets the following requirements:

    • Strip length: 7.5 mm to 10 mm (0.295 in. to 0.394 in.)
    • Gauge: 24 AWG to 16 AWG
    • Conductor type: solid or multi-stranded
      Note If you are using multi-stranded cabling, twist the strands together before insertion. NI recommends stripping and tinning multi-stranded conductors before insertion for additional reliability.

    For generic cabling, use the safety interlock input connector included in the kit for your instrument to join the cabling to the instrument front panel(s). Identify the pins of the connector with the following figure.

    Figure 23. Safety Interlock Input Connector Pinout

    1378

    Complete the following steps to connect the safety interlock subsystem with generic cabling.

    1. Ensure the AC power source is connected to the chassis before installing the connector.
      The AC power cord grounds the chassis and protects it from electrical damage.
    2. Power off the chassis.
    3. Touch any metal part of the chassis to discharge static electricity.
    4. Prepare and connect a cable of sufficient length to the system safety relay you have designed for your interlock implementation.
      1. Measure and mark your strip length on the cable.
      2. Use an insulation strip tool to strip back the insulation to the appropriate length.
      3. Wire the cable to the test system enclosure at the user access points as instructed in any documentation for the enclosure.
    5. Strip and connect the other end of the cable to the safety interlock input connector included in the kit for your instrument.
      1. Connect the red safety interlock signal wire of the cable into pin 1 of the interlock connector.
      2. Connect the black safety interlock ground wire of the cable into pin 2 of the interlock connector.
    6. To extend your safety interlock circuit to additional instruments, connect cabling of sufficient length to the interlock connector in the previous step.
      1. Connect the red safety interlock signal wire from a second interlock cable into pin 3 on the interlock connector of the first cable.
      2. Connect the black safety interlock ground wire from a second interlock cable into pin 4 on the first interlock connector.
      3. Repeat with additional cables until you have connected enough safety interlock connectors for as many instruments as you need.
      Figure 24. Safety Interlock Pass-Thru Connection

      1378

    7. After inserting all of the cables, inspect for loose strands.
    8. Tighten any retention screws on the safety interlock input connectors to hold the cabling in place.
    9. Attach the safety interlock input connectors to the instruments.
    10. Power on the chassis.
    Once you have properly connected safety interlock cabling within your system, continue with Testing the Safety Interlock System.

    Testing the Safety Interlock

    To ensure safe operation of an NI SMU with a safety interlock, you should periodically test the safety interlock for proper functionality. The recommended test interval is at least once per day of continuous usage.

    This procedure assumes you have already integrated your instrument into a system that makes use of the safety interlock.

    You can conduct the test using either the NI-DCPower API in your environment of choice or InstrumentStudio.

    Complete the following steps to verify that the safety interlock of your NI SMU is functioning properly.

    1. Disconnect the output connector assembly from the instrument front panel.
    2. Ensure that the safety interlock input on the test fixture is closed.
      Refer to the Determining Safety Interlock Status for instructions on how to make this determination.
    3. Configure the test settings.
      OptionDescription
      NI-DCPower API
      1. Set Configure Output Function to DC Voltage.
      2. Set the voltage level range to 200 V with Configure Voltage Level Range.
      3. The voltage level to 42.4 V with Configure Voltage Level.
      4. Set the current limit range to 1 mA with Configure Current Limit Range.
      5. Set the current limit to 1 mA with Configure Current Limit.
      6. Set the sense to Local with Configure Sense.
      InstrumentStudio
      1. Set the output to Voltage using the dropdown menu for the channel.
      2. Click 1378 for the channel.
      3. Set Voltage level range to 200 V.
      4. Set Voltage level to 42.4 V.
      5. Set Current limit to 1 mA.
      6. Set Current limit range to 1 mA.
      7. Ensure Sense is set to Local.
      8. Ensure Output is set to On.
    4. Begin voltage generation.
      • NI-DCPower: Call Initiate With Channels.
      • InstrumentStudio: Click Run.
      The Voltage status indicator on the instrument front panel should be amber.
    5. Open the safety interlock input circuit using the test fixture.
      The Voltage status indicator on the instrument front panel should be red, and you should receive a software error.
    6. Close the safety interlock input circuit using the test fixture.
    7. Clear the error condition and reset the instrument to its default state.
      • NI-DCPower API: Call Reset With Channels.
      • InstrumentStudio: Reset the instrument with MAX.
      The Voltage status indicator on the instrument front panel should be green.
    The instrument passes the safety interlock test if the Voltage status indicator operates as described.

    Continue to test your safety interlock system according to the recommended interval.

    Caution If the instrument fails the safety interlock test, discontinue use of the instrument and contact an authorized NI service representative to request a Return Material Authorization (RMA).

    Determining Safety Interlock Status

    You can verify the status of the safety interlock circuit on a particular NI SMU programmatically with the NI-DCPower API or by the color of an LED on the instrument front panel.

    Check the safety interlock status in either of the following ways.
    • Programmatic: Read Interlock Input Open. When True, the interlock circuit is open, which means the interlock is limiting voltage output.
    • Visual: Check the color of the Voltage LED indicator on the instrument front panel. Refer to the front panel documentation for your instrument to interpret the meaning of the different LED colors.

    Resolving Open Safety Interlock Software Errors

    When the safety interlock is properly integrated, it is possible to program voltage levels and limits greater than those allowed by the interlock when it is open. When this occurs, NI-DCPower prevents a channel from actually sourcing this voltage and, in some cases, generates an error or shuts down the channel.

    Complete the following steps to resolve a safety interlock error:
    1. Identify and fix the underlying cause of the software error.
    2. Clear the error.
      • NI-DCPower API: Call Reset With Channels for the affected channels.
      • InstrumentStudio: Reset the instrument with MAX.